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Chapter 7

Chapter 7. Active Fish Capture Methods. 7.1 Introduction. Moving gear/nets through water Collecting Fish Crustaceans & Other Inverts. 3 Main Gear Types. Towed Nets Dredges Surrounding Nets Plus Others (Hook and line, cast nets). Standardization of effort. Pull trawl fixed time

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Chapter 7

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  1. Chapter 7 Active Fish Capture Methods

  2. 7.1 Introduction • Moving gear/nets through water • Collecting • Fish • Crustaceans & Other Inverts.

  3. 3 Main Gear Types • Towed Nets • Dredges • Surrounding Nets • Plus Others (Hook and line, cast nets)

  4. Standardization of effort • Pull trawl fixed time • Sweeping specific area

  5. Requirements • Larger boats • More manpower • Less sampling time than passive gear

  6. 7.2 Net Material and Construction • Natural Materials - cotton, hemp, linen • Thick, heavy • Rotting is a problem • Synthetic materials - polyethylene, polypropylene • Stronger, thinner • Less prone to decay

  7. Mesh size (cont.) • small fish pass through mesh, measure consistently

  8. Mesh size • Bar length - distance knot to knot along diagonal • Stretch measure - knot to knot distance when mesh is stretched

  9. Hanging ratio (E) • E = rope length / stretched length of netting • Or use hanging % = 100 * ( 1 - E ) • Range for trawls (E = 0.6 - 0.8)

  10. 7.3 Dragged or Towed Gears: Trawls • Funnel-shaped with cod-end (narrow backend) • Midwater or bottom • Beam or otter trawls

  11. Beam Trawls • Fixed width • Sweep fixed area consistently • Somewhat cumbersome if beam is large

  12. Otter Trawls • Otter boards or trawl doors use force of water • Oval or square, reinforced skids • Boards hold open net • Mouth width depends on force (inconsistent)

  13. Trawling advantages • Fish in good condition (unless deep trawls... pressure changes) • For release of live specimens, short trawls (5-15 min) • Quantitative index of pop abundance

  14. Trawl disadvantages • Can't sample when bottom is irregular (stobs, rocks) • Need powerful boat (40 hp or greater) • By-catch of other species?

  15. Examples of Sampling Programs • MARMAP, SEAMAP - National Marine Fisheries Service • Eastern Seaboard • Yankee 36 trawl (18 m headrope, 24 m footrope) • Transects perpendicular to shore out to cont. shelf

  16. Examples of Sampling Programs • Great Lakes Fisheries Survey - Great Lakes Laboratory • Bottom & midwater trawls & acoustic surveys • 5 - 150 m depth • seasonal variation

  17. Bottom Trawl Modifications • Rollers on the sweep chain • Tickler chains on the sweep chain • Plastic strips on the bag to prevent snagging • Size & material of doors

  18. Midwater Trawls (cont.) • Mesh at mouth coarse, mesh finer toward cod-end • Four seams

  19. Midwater Trawls • Depth determined by boat speed and warp out • Determined by angle or by pressure sensor on net

  20. Use of Midwater Trawls • Sample pelagic fish • Ground truthing for acoustic surveys • Sampling larvae and juveniles (1 mm mesh)

  21. Examples • Remote Midwater Trawl • Isaacs-Kid Midwater Trawl • MOCHNESS • BIONESS

  22. Evaluating Gear Performance • Crossed or twisted trawl doors? • Did net catch fish? • Net hang-up on bottom? • Cod end tied?

  23. Technology to Evaluate Gear • Depth/Pressure sensors • Laser distance measures • Video camera mounted on gear

  24. 7.4 Dragged or Towed Gears - Dredges • Heavy metal frames • Chain link bags • Cutting bars or teeth dig into substrate

  25. Scallop dredge • Rectangular metal opening • Triangular frame attached to single warp • Bag of metal rings lined with smaller mesh net • Pressure plate to force dredge to dig into substrate • Oyster dredge is higher, with a shorter bag

  26. Monitoring performance • Temperature/depth sensors • Check the bottom of the dredge, abrasion will shine up the metal • Debris (rocks, wood) in the dredge usually means catch will be low

  27. Examples of dredge surveys (cont.) • 1. NMFS sea scallop survey • Along East Coast • 2.44-m wide • 5.1 cm diameter rings • 3.8 cm polypropylene mesh liner

  28. Examples of dredge surveys • 2. DFO survey of Georges Bank

  29. 7.5 Surrounding or Encircling Gear • beach seines, purse seines, lampara net • trap fish inside fence of mesh • area sampled is fairly standard

  30. Seine components (cont) • Float line - cork, styrofoam, or plastic floats hold mesh upright • Lead line - lead weights attached or lead in core of polypropylene line

  31. Seine components • Bunt - section of mesh wall where fish are concentrated • Bag - small pocket sewn into the bunt for further fish concentration • Mesh - forms the wall of the seine.

  32. Beach or haul seine • long regular wall of mesh • with bunt (& maybe bag) • walk wings around fish • retrieve, but ensure lead line (mudline) stays on bottom

  33. Fishing a beach seine (cont.) • fished near shore by wading; no obstructions to lift lead line • set in semi-circle; retreive both ends or • set perpendicular to shore, walk along; then offshore fisher curls to shore

  34. Fishing a beach seine • 1-3 wings or leaders (guide fish) • enclosure with throat • float • anchor • pay attention to capture efficiencies - vary diel, seasonal, by species

  35. Purse seines • For pelagic (open water) species

  36. Purse seines • Or demersal if leadline goes to the bottom • Can fish with one or two boats

  37. Fishing a purse seine (cont.) • Wall of mesh encircles fish • Pull purse line from one or both ends • Bottom of net cinches shut like drawstring purse

  38. Fishing a purse seine • Fish are in a bowl of mesh • Bowl is made smaller until fish in bunt of seine

  39. Examples • 1. Juvenile coho salmon - Oregon and Washington • 495-m long seine set in transects up & down coast • catch showed juveniles migrate north in ocean

  40. Examples (cont.) • 2. Rainbow trout - Lake Washington • 600-m long, 37-m deep, 25mm stretch mesh netting • collect fish for food habit study

  41. Lampara net • For catching fish near surface • Used over rough bottom where beach or purse seine won't work • Leadline shorter than float line • After circling fish, ends of leadline pulled • Leadlines come together making a bowl full of fish

  42. 7.6 Other Active Sampling Gears (cont.) • Push nets • Rectangular rigid frame with mesh behind • Pushed in front of small boats - sample fish fry

  43. 7.6 Other Active Sampling Gears (cont.) • Neuston nets • push net towed to the side or behind a boat

  44. 7.6 Other Active Sampling Gears (cont.) • Lift nets • three line bridle on a bowl of mesh • bait the mesh or attract fish over net with light • lift the bowl and trap the fish (or crabs)

  45. 7.6 Other Active Sampling Gears (cont.) • Pop nets • Rectangular frame of mesh • Set on bottom • Released to pop up and form a box

  46. 7.6 Other Active Sampling Gears (cont.) • Dip nets • Circular net on a pole • Lift fish from water - during electrofishing • Remove fish from containers

  47. Other Active Sampling Gears (cont.) • Fish Wheel • Ferris wheel for fish • Native Americans harvest anadromus fish this way

  48. Other Active Sampling Gears (cont.) • Cast nets (requires skill) • Circle of mesh • Weighted edges • Draw-string for cinching net closed • Usually near-shore for bait fish

  49. Other Active Sampling Gears (cont.) • Drop nets • Rigid cylinder or box of mesh (usually <1m^2) • Thrown or dropped in sample area • Fish removed from fixed area… quantitative sample

  50. Other Active Sampling Gears (cont.) • Angling • Rod and reel sampling • To collect brood stock • To collect fish in good shape for radio telemetry studies • When other gears won't work

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